1. Field of the Invention
The present invention relates to a nonlinear device, for example, used for driving an active matrix type liquid crystal display apparatus.
2. Description of the Related Art
As the above-mentioned nonlinear device, a thin film transistor (TFT) which is a three-terminal device, a thin film diode (TFD) which is a two-terminal device, and the like are known. In particular, the two-terminal device has a simpler structure in which a nonlinear resistant layer is provided between a pair of electrodes and requires fewer masks for production, compared with the three-terminal device such as a TFT. Thus, the two-terminal device holds promise of reducing production cost.
At present, nonlinear devices using tantalum oxide (Japanese Patent Publication No. 61-32674), silicon nitride or silicon oxide (Japanese Patent Publication No. 6-17957), zinc sulfide (Japanese Laid-Open Patent Publication No. 7-134315), etc. for nonlinear resistant layers have been developed. Particularly, in nonlinear devices using zinc sulfide for a nonlinear resistant layer, the nonlinearity of I (current)-V (voltage) characteristics is large, and the I-V characteristics can be controlled by doping impurities into the zinc sulfide. Therefore, such a nonlinear device has the advantage in that the I-V characteristics can be designed in accordance with the electro-optic characteristics of a display medium. Thus, a display apparatus with a high contrast can be realized by using, as a switching device, a nonlinear device which uses zinc sulfide for a nonlinear resistant layer.
FIG. 3 is a cross-sectional view of a conventional nonlinear device using zinc sulfide for a nonlinear resistant layer. This nonlinear device includes a first electrode 52 having a predetermined shape on an insulating substrate 51 and a nonlinear resistant layer 53 composed of a material mainly containing zinc sulfide formed so as to cover the first electrode 52. An interlevel insulator 54 having a contact hole 54a is formed on the nonlinear resistant layer 53, and a second electrode 56 is formed on the interlevel insulator 54 so as to fill the inside of the contact hole 54a. The second electrode 56 is in contact with the nonlinear resistant layer 53 in the contact hole 54a of the interlevel insulator 54. This nonlinear device has a structure in which the nonlinear resistant layer 53 is in contact with the first and second electrodes 52 and 56.
Hereinafter, the case using aluminum, for example, as a material for the above-mentioned second electrode 56 will be considered. In the case where the temperature of the nonlinear device increases when the nonlinear resistant layer 53 is mainly made of zinc sulfide and the second electrode 56 is mainly made of aluminum being in contact with each other, aluminum reacts with sulfur to generate aluminum sulfide. Aluminum sulfide is generated for the following reason: the standard free energy .DELTA.G.sub.Al of the reaction, in which aluminum reacts with sulfur to generate a sulfide, has a negative value lower than the standard free energy .DELTA.G.sub.Zn of the reaction in which zinc reacts with sulfur to generate a sulfide.
Hereinafter, the reaction of the generation of aluminum sulfide will be described in terms of thermodynamics.
Whether or not the reaction spontaneously proceeds under the given condition is determined by the sum .DELTA.S.sub.T of the entropy change .DELTA.S.sub.sys of a reaction system and the entropy change .DELTA..sub.sur of the surroudings of the reaction system represented by the following equation (1). EQU .DELTA.S.sub.T =.DELTA.S.sub.sys +.DELTA.S.sub.sur ( 1)
According to the Second Law of Thermodynamics, the reaction spontaneously proceeds at .DELTA.S.sub.T &gt;0. Here, the spontaneous reaction under constant pressure is considered. Under constant pressure, the following equation (2) holds. EQU .DELTA.S.sub.sur =-(.DELTA.H/T) (2)
where H is the enthalpy, and T is the temperature.
It is understood from the equations (1) and (2) that the following equation (3) should hold so as to allow the reaction to spontaneously proceed. EQU .DELTA.H-T.DELTA..sub.sys &lt;0 (3)
.DELTA.H-T.DELTA.S.sub.sys in the equation (3) is known as Gibbs free energy .DELTA.G. The reaction spontaneously proceeds at .DELTA.G=.DELTA.H-T.DELTA.S.sub.sys &lt;0.
For example, the reaction between zinc and aluminum can be represented as follows. The standard free energy .DELTA.G.sub.Al of the reaction in which aluminum reacts with sulfur to generate a sulfide at room temperature is about -130 kcal, and the standard free energy .DELTA.G.sub.Zn of the reaction in which zinc reacts with sulfur to generate a sulfide at room temperature is about -110 kcal. Thus, these reactions can be represented by the following equations (4) and (5). EQU (4/3)Al+S.sub.2 =(2/3)Al.sub.2 S.sub.3 .DELTA.G=-130 kcal (4) EQU 2Zn+S.sub.2 =2ZnS .DELTA.G=-110 kcal (5)
The standard free energy .DELTA.G of the reaction in which aluminum is brought into contact with zinc sulfide can be obtained from the equations (4) and (5). The result is represented by the following equation (6). EQU (4/3)Al+2ZnS=(2/3)Al.sub.2 S.sub.3 +2Zn .DELTA.G=-20 kcal (6)
It is understood from the equation (6) that the change in standard free energy is -20 kcal.
As described above, when the standard free energy .DELTA.G of the reaction has a negative value, the reaction proceeds spontaneously. As .DELTA.G has a smaller negative value, the reaction is more likely to proceed. Therefore, in the case where zinc sulfide is in contact with aluminum, zinc sulfide is desulfurized to become zinc, and aluminum reacts with sulfur to generate aluminum sulfide.
In the nonlinear device used as a switching device, large nonlinearity of I-V characteristics, satisfactory symmetry of I-V characteristics, stable I-V characteristics, and the like are required.
When considering the nonlinear device using zinc sulfide for a nonlinear resistant layer for the above-mentioned characteristics, the nonlinearlity of I-V characteristics is large. Therefore, such a nonlinear device is considered to be suitable as a switching device. However, when highly reactive zinc sulfide is used for the nonlinear resistant layer, sulfur contained in zinc sulfide may react with an electrode material. As a result, there is a problem that the state of the interface between the nonlinear resistant layer and the electrode tends to change, rendering I-V characteristics to vary over time.
FIG. 4 shows an example of change in I-V characteristics over time of the nonlinear device as described above. FIG. 4 shows I-V characteristics of a nonlinear device using zinc sulfide doped with nickel for a nonlinear resistant layer and aluminum for an electrode. As is understood from FIG. 4, in this nonlinear resistant layer, I-V characteristics 32 after aging are shifted toward a lower voltage side, compared with initial I-V characteristics 31.
When a nonlinear device in which I-V characteristics change over time is used as a switching device of a display apparatus, the I-V characteristics are shifted toward a lower voltage side over the passage of operation time of the display apparatus. This allows liquid crystal to be driven on a lower voltage side, resulting in a decrease in a display contrast, etc.